1. Field of the Invention
This invention relates to a terminal coupled to an integrated services digital network (ISDN), and more particularly to a terminal coupled to the ISDN having a operation for executing a confirmation call for detecting a change in a destination terminal of a communication procedure from a group III (G3) facsimile procedure to a group IV (G4) facsimile procedure.
2. Discussion of the Background
A public switched telephone network (PSTN) is not suitable for transmitting digital data, such as, digital voice data for communications, digital image data for a facsimile, digital video image data, and so forth. Since a demand for such digital data is rapidly increasing, another data transmission network, namely an integrated services digital network (ISDN), is designed to handle digital data and to replace the PSTN. For the time being, however, until the PSTN is completely replaced by the ISDN, the ISDN and the PSTN are provided to allow two terminals, which have the same communication procedure and which are respectively coupled to the different networks, to exchange data with each other. However, one terminal which is coupled to the ISDN and transmits and receives image data under a communication procedure conforming to a group 4 (G4) facsimile procedure prescribed by CCITT (Comite Consultif International Telegraphique et Telephonique) recommendations, such as, a G4 facsimile, for example, can not exchange image data with another terminal when the another terminal is coupled to the PSTN and transmits and receives image data under a communication procedure conforming to a group III (G3) facsimile procedure prescribed by CCITT recommendations, such as, a G3 facsimile.
In this circumstance, most G4 terminals additionally include a function of a G3 facsimile procedure to have an internetworking function. At the same time, the G4 facsimile terminal, for example, requires a function for discriminating whether a different terminal to be called is a G4 facsimile coupled to the ISDN or a G3 facsimile coupled to the PSTN. Several proposals responding to such a demand are disclosed in the official gazette for Laid Patent Applications TOKUKAI SHO 63-8664, TOKUKAI HEI 2-21758, and so forth, for example.
Generally, the G4 facsimile also includes a memory for storing information of different terminals to be called, such as, for example, a calling number, a short formed calling number to be used in an abbreviated dialing mode (ADM), a corresponding identification for identifying whether the different terminal to be called conforms to the G3 facsimile procedure or the G4 facsimile procedure, and the like, to facilitate a process of setting up a call.
However, the identification to be memorized, as described above, does not always correctly represent the communication procedure to which the corresponding different terminal to be called conforms. The reason is that an operator may not know what communication procedure the corresponding different terminal to be called actually conforms to at a time when the operator inputs the identification into a memory of the calling terminal at the operator's site. Also, an operator may not know when a different terminal to be called has been changed to a G4 facsimile after the operator has correctly input an identification of the different terminal to be called as a G3 facsimile machine. Thus, a discrepancy is created between a memorized identification and an actually available communication procedure, and cases with and without such a discrepancy are explained below.
In a first case, a discrepancy is created such that an identification in the memory of a calling G4 facsimile incorrectly indicates that a different terminal to be called conforms to the G4 facsimile procedure, despite the fact that the different terminal to be called is coupled to the PSTN and conforms only to the G3 facsimile procedure. In this case, when the G4 facsimile sets up a call to the different terminal using, for example, a short formed calling number memorized in the memory of the G4 facsimile, the call is set up under the G4 facsimile procedure in accordance with the memorized identification. As a result, the calling G4 facsimile fails in connecting to the called different terminal. However, upon a detection of the failure, the calling G4 facsimile immediately changes its communication procedure from the G4 facsimile procedure to the G3 facsimile procedure and sets up another call under the G3 facsimile procedure. Then, the calling G4 facsimile succeeds in connecting to and transmitting data to the called different terminal with the G3 procedure. After that, the calling G4 facsimile automatically changes the identification in its memory to indicate that the called different terminal conforms to the G3 facsimile procedure; this updated identification is then used for setting up future calls.
In a second case, a stored identification indicates that a different terminal to be called conforms to the G4 facsimile procedure, and the different terminal to be called actually conforms to both the G3 facsimile procedure and the G4 facsimile procedure. In this case, the G4 facsimile sets up a call under the G4 facsimile procedure in accordance with the stored identification, and there is no problem in connecting to and transmitting data to the called different terminal smoothly by the G4 procedure.
In a third case, a stored identification indicates that a different terminal to be called conforms to the G3 facsimile procedure, and the different terminal to be called actually conforms only to the G3 facsimile procedure. In this case, the G4 facsimile sets up a call under the G3 facsimile procedure in accordance with the identification and has no problem in connecting to and transmitting data to the called different terminal smoothly. Once the G4 facsimile sets up a call in the above-mentioned first case, the first case turns into this third case because the G4 facsimile changes its stored identification to indicate that the called different terminal conforms to the G3 facsimile procedure and the discrepancy is dismissed as described in reference to the first case.
In a fourth case, a stored identification indicates that a different terminal to be called conforms to the G3 facsimile procedure, and the different terminal to be called conforms to both the G3 facsimile procedure and the G4 facsimile procedure. In this case, the G4 facsimile sets up a call under the G3 facsimile procedure in accordance with the stored identification. As a result, the calling G4 facsimile succeeds in connecting to and transmitting data to the called different terminal under the G3 facsimile procedure, despite the fact that the called different terminal also conforms to the G4 facsimile procedure.
From the above cases, it is evident that a G4 facsimile is capable of automatically switching its communication procedure from the G4 facsimile procedure to the G3 facsimile procedure when it detects that the different terminal to be called does not conform to the G4 facsimile procedure, since communication does not take place with the G4 procedure. Further, in the fourth case, since communication does take place in the G3 procedure, the G4 facsimile does not switch to the G4 facsimile procedure from the G3 facsimile procedure during setting up a call even when the different terminal to be called conforms to the G4 facsimile procedure.
However, a situation may arise in which a different terminal conforming only to the G3 facsimile procedure in the above-mentioned first case and third case is replaced or updated by another different terminal conforming to both the G4 facsimile procedure and the G3 facsimile procedure at a time in the future, and then, the case becomes the same as the above-mentioned fourth case in which a calling G4 facsimile does not switch its communication procedure during setting up a call from the G3 facsimile procedure to the G4 facsimile procedure, even through the called different terminal conforms to the G4 facsimile procedure.
Further, each time when an operator for the G4 facsimile is notified by any means that a communication procedure of the different terminal is changed from the G3 facsimile procedure to the G4 facsimile procedure, the operator needs to manually change a corresponding identification in the memory to indicate that the different terminal conforms to the G4 facsimile procedure.
A cause for this inconvenience on a G4 facsimile is, simply, that the G4 facsimile is not structured to detect whether or not the different terminal to be called conforms to the G4 facsimile procedure, once a communication with the G3 procedure is established with the called different terminal, despite the fact that the called different terminal has been updated to conform also to the G4 facsimile procedure.